MUSEUM OF COMPARATIVE ZOOLOGY. 61 



ucciput, and sending another branch between the eye and the spiracle 

 toward the mouth. Between the lip and the eye, on the lower aspect, 

 the latter branch sends another backward past the angle of the mouth, 

 and, farther on its way forward, sends still another behind each nostril, 

 itself usually joining the main canal at the end of the snout. Com- 

 monly a branch from that passing the angle of the jaws extends under 

 the jaw, behind the mouth, toward the middle of the chin ; in some cases 

 this branch is disconnected ; in others it does not appear ; occasionally 

 it is continuous across the symphysis. The branch behind the nostril 

 passes toward a similar one from the opposite side of the head, either 

 uniting with it or approaching it closely for a short distance under the 

 base of the rostral cartilage, after which the two diverge slightly and 

 continue forward to the end of the rostrum. On the top of the skull 

 the tubes are more or less strongly attached to the cartilage, in troughs 

 or depressions in which they are often deeply seated. Under the base 

 of the rostrum there is also a firm attachment to the cartilage. Else- 

 where the canals lie at varying depths in the skin or below it. Gener- 

 ally they are tubes with openings to the surface through simple to 

 many-branched tubules. On species of Heptabranchias and on Chlamy- 

 doselachus they appear, in great part of their extent, as open dermal 

 grooves. On one of the genera of the Holocephala they are open 

 furrows, on the other they are tubes. 



NAMES OF THE CANALS. 



Plate I. 



The most convenient designations for the different canals, or parts of 

 canals, are those derived from the names of the portions of the body 

 traversed by them, or from those of the organs near which they pass. 

 The propriety of the names cephalic for all the canals of the head, and 

 corporal for all those of the body, is at once apparent. Their position 

 along the flanks makes the name laterals (I), by which they are gener- 

 ally known, a very appropriate one for the two main corporals. In the 

 Batoidei there is a canal peculiar to them extending out upon the pec- 

 toral fin; this may be called the pleural (p). On the lower surface 

 'it becomes a subpleural. The areas enclosed by the pleural tubes are 

 the pleural areas ; those sometimes enclosed by scapular tubules on 

 the shoulder are called the scapular areas. 



Running longitudinally on the top of the head are the two principal 

 cephalic tubes, the cranials (cr) ; anteriorly, on the rostrum they be- 



